1. Field of the Invention
This invention relates to a printed wiring board having at least one rigid portion and at least one flexible portion. In addition, this invention relates to a method of fabricating a printed wiring board having at least one rigid portion and at least one flexible portion.
2. Description of the Related Art
In recent years, there has been progress in downsizing electronic apparatuses and their multifunction properties. Furthermore, there have been increases in the rates of signal processing by electronic apparatuses. For advanced electronic apparatuses, it is desirable to mount electronic components on printed wiring boards at high densities.
A printed wiring board having a rigid portion and a flexible portion is referred to as a flexible rigid printed wiring board. Generally, a flexible rigid printed wiring board can be made into a desired shape. Therefore, the flexible rigid printed wiring board is useful to design a compact electronic apparatus.
The use of a flexible rigid printed wiring board in an electronic apparatus removes a connector which would be required for electric connection between separate substrates (circuit boards). The removal of the connector increases a usable space in the apparatus, and decreases a wiring length. The decreased wiring length is advantageous to a higher rate of signal processing. After electronic components are mounted on the flexible rigid printed wiring board, the board can be bent into a desired three-dimensional shape. Accordingly, the flexible rigid printed wiring board allows free layout of parts in the apparatus, and hence enables the apparatus to be downsized.
Flexible rigid printed wiring boards are of two types called “type A” and “type B” respectively.
A type-A board includes a flexible substrate, and a rigid substrate bonded to a portion of a surface of the flexible substrate. The type-A board may include a flexible substrate, and rigid substrates bonded to portions of two surfaces of the flexible substrate. An example of the rigid substrate or substrates is “FR-4”.
A type-B board includes a flexible substrate having a portion provided with a multilayer structure. The multilayer portion is properly rigid so that a desired wiring pattern can be formed thereon.
It is known to form buildup layers on one or two surfaces of a rigid portion of a type-A board or a type-B board. The buildup layers constitute a multilayer structure.
Japanese patent application publication number 04-034993/1992 shows a type-A board. Specifically, Japanese application 04-034993/1992 discloses the manufacture of a multilayer printed wiring board having a flexible portion. According to Japanese application 04-034993/1992, a flexible printed wiring board uses a flexible polyimide film as a base film. The flexible polyimide film has a circuit pattern formed as follows. Copper foil is affixed to a surface of an original polyimide film. The copper foil is etched to form the circuit pattern. A first hard printed wiring board, a first adhesive sheet, a circuit protective sheet, the flexible printed wiring board, a second adhesive sheet, and a second hard printed wiring board are sequentially superposed into a laminate. The laminate is pressed and heated so that the layers therein are bonded together to form a single body.
In Japanese application 04-034993/1992, the circuit protective film includes a polyimide film, and an acrylic-epoxy adhesive layer provided on a surface of the polyimide film which will adjoin the flexible printed wiring board. Each of the first and second adhesive sheets includes a polyimide film, and acrylic-epoxy adhesive layers provided on two surfaces of the polyimide film respectively. Each of the adhesive sheets has a previously-formed opening at a place corresponding to a flexible portion of an end product, that is, a multilayer printed wiring board. Each of the first and second hard printed wiring boards includes a glass-imide resin plate, and copper foil affixed to a surface of the glass-imide resin plate which will be an outer surface of the end product. The other surface of the glass-imide resin plate, which will adjoin the first or second adhesive sheet, has a previously-formed groove in alignment with the opening in the related adhesive sheet.
In Japanese application 04-034993/1992, after the layers in the laminate are bonded together, the copper foil of each of the first and second hard printed wiring boards is etched and portions of the hard printed wiring boards which define through holes are plated to form desired circuit patterns thereon. Then, the outer surfaces of the hard printed wiring boards are provided with openings which reach the grooves therein. The openings in the hard printed wiring boards, and the openings in the adhesive sheets cause the flexible portion of the end product.
Japanese patent application publication number 05-090757/1993 shows a type-B board. Specifically, Japanese application 05-090757/1993 discloses a multilayer flexible printed wiring board including a first multilayer portion, a second multilayer portion, and a flexible portion extending between the first and second multilayer portions. Each of the multilayer portions has a laminate of an upper flexible printed wiring board, an upper adhesive sheet, an intermediate flexible printed wiring board, a lower adhesive sheet, and a lower flexible printed wiring board which are sequentially arranged in that order. The flexible portion has a laminate of an upper flexible printed wiring board, an adhesive sheet, and a lower flexible printed wiring board which are sequentially arranged in that order. The upper flexible printed wiring board in the flexible portion is integral with the upper flexible printed wiring boards in the multilayer portions. The adhesive sheet in the flexible portion is integral with the upper adhesive sheets in the multilayer portions. The lower flexible printed wiring board in the flexible portion is integral with the intermediate flexible printed wiring boards in the multilayer portions.
The multilayer flexible printed wiring board in Japanese application 05-090757/1993 has a recess located between the first and second multilayer portions. The bottom of the recess is defined by the flexible portion. Thus, the flexible portion is thinner than the multilayer portions.
To fabricate the multilayer printed wiring board (the type-A board) in Japanese application 04-034993/1992, it is necessary to previously make a hard printed wiring board and a flexible printed wiring board by different steps respectively. Then, first, second, and third steps are required. The first step is to bond the hard printed wiring board and the flexible printed wiring board together by using an adhesive layer. The second step is to provide through holes for electrical connection between separate conductor layers. The third step is to pattern a surface-layer conductor with a circuit. Accordingly, a relatively large number of steps are required to complete the multilayer printed wiring board. It is necessary to use an installation for making hard printed wiring boards and also an installation for making flexible printed wiring boards. Therefore, the cost of the production of the multilayer printed wiring board is relatively high.
In the multilayer printed wiring board of Japanese application 04-034993/1992, the material for the hard printed wiring boards, the material for the flexible printed wiring board, and the material for the adhesive layers differ from each other, and have different coefficients of thermal expansion. The different thermal expansion coefficients have a chance of causing the following problems in the presence of a temperature load or a temperature change. Conditions of the connection of plating to the walls defining through holes are made uneven. Exfoliation occurs in connecting portions. Wiring patterns in different layers are misaligned. Warp of the multilayer printed wiring board is relatively great.
As previously mentioned, the multilayer flexible printed wiring board (the type-B board) in Japanese application 05-090757/1993 includes the flexible printed wiring boards and the adhesive sheets. In general, material for such flexible printed wiring boards and material for such adhesive sheets differ from each other, and have different coefficients of thermal expansion. The different thermal expansion coefficients have a chance of causing the following problems in the presence of a temperature load or a temperature change. Conditions of the connection of plating to the walls defining through holes are made uneven. Exfoliation occurs in connecting portions. Wiring patterns in different layers are misaligned. Warp of an end-product board is relatively great.
The multilayer flexible printed wiring board (the type-B board) in Japanese application 05-090757/1993 tends to be considerably lower in hardness than a typical rigid board. Therefore, in some cases, it is necessary to support one side of the multilayer flexible printed wiring board by a suitable plate when electronic components are mounted on the other side of the board.